Update: Flu Season 1999-2000

Summary
During the past 6-8 weeks, the proportion of patients seen by a sentinel network of physicians
for influenza-like illness, the number of states and territories reporting
"widespread" and "regional" influenza activity, and the numbers and types
of circulating influenza viruses are similar to the previous two seasons (1997-98 and
1998-99). These findings suggest that this year's flu season has not been unusually severe.
However, the reported deaths from pneumonia and influenza (P&I) deaths are higher this
year. The causes of high P&I mortality levels are uncertain, but these figures should be
interpreted with caution since they are preliminary and, in part, reflect changes made this
year to the P&I reporting case definition. In addition, there is the possibility that
other factors, such as other respiratory infections, could be contributing to the increased
P&I mortality.

Background information on CDC's influenza surveillance systems
Each year, CDC actively monitors the flu season through several surveillance systems. These
systems: 1) track the number (and percentage) of respiratory specimens testing positive for
influenza virus and the numbers of influenza isolates identified by a group of >100
laboratories in the U.S.; 2) track the number of patient visits for influenza-like illness to
a group of approximately 400 sentinel physicians located throughout the U.S.; 3) track state
and territorial influenza activity levels as estimated and reported by the state or
territorial epidemiologist; and 4) track the number of deaths in 122 cities attributed to
pneumonia and influenza (P&I deaths). In addition, CDC receives reports about local
outbreaks from state health departments and other sources.

During December through mid-January of the current 1999-2000 flu season, the percentages of
respiratory specimens testing positive for influenza, the numbers and types of influenza
viruses that were isolated and characterized, the numbers of patients visiting sentinel
physicians for influenza-like illness, and the numbers of states and territories reporting
widespread or regional influenza activity were similar to levels seen in the past two years.
All three seasons have been predominated by influenza A(H3N2) Sydney-like viruses. Since
P&I mortality levels typically lag behind virus and clinical illness activity markers by
about 2 or 3 weeks, CDC 's earlier assessments of influenza activity levels were based on the
levels of viral isolations and clinical illness activity.

During the week ending January 15, the percentage of P&I deaths in the 122 cities
system reached 10.5%, which is higher than is usually seen. This level could indicate that
this year's flu season is more severe than usual. However, before this can be concluded, some
other important considerations must be taken into account.

First, the 122 cities mortality system is a rapid mortality monitoring system and its
results are considered preliminary. The final assessment of an influenza season's severity
is based on complete national death data, which usually are not available for analysis
until 2 to 3 years after a calender year.

Second, a somewhat broader reporting case definition was implemented in the 122 cities
mortality system for this flu season. This is an important change in the methods that
could contribute to an increase in this season's P&I mortality estimates. Because the
impact of the case definition change is uncertain, CDC has cautioned since the early part
of the 1999-2000 season that the P&I figures must be interpreted with caution.

Third, as is true every year, levels of P&I mortality can be influenced by several
other factors in addition to influenza. These factors include simultaneous circulation of
other respiratory pathogens, such as respiratory syncytial virus. Higher levels of such
pathogens could contribute to higher than usual levels of P&I mortality.

Fourth, it is important to note that 3 of the 4 surveillance systems did not indicate
that this was an unusually severe year.

Can you put this year's P&I mortality level into context. How much higher than
usual is this level of P&I mortality?
P&I mortality levels can vary substantially from season to season. During the previous
three flu seasons (1996-97 through 1998-99), all of which were predominated by influenza
A(H3N2) viruses, P&I mortality levels in the 122 cities system peaked at 9.1% in the
1996-97 season, 9.0% in the 1997-98 season, and 8.8% in the 1998-99 season.

What is CDC doing to determine whether this was a real increase in P&I mortality
and if the season was more severe than usual?
Similar to what is done for every season, complete national mortality data will be analyzed
when they are made available. This is usually 2-3 years after a calender year. Laboratory work
on characterizing this years influenza viruses will continue. CDC will work with state and
local health departments to investigate unusual outbreaks. Finally, CDC will be working with
investigators to analyze data from managed care organizations to determine vaccine
effectiveness estimates for this season. These results usually are not available until the
late summer.

Why are both pneumonia and influenza deaths monitored? Why not influenza deaths alone?
Most deaths from influenza result from complications of this infection, including bacterial
pneumonia. Following changes in pneumonia and influenza deaths is a common approach for
monitoring influenza-related mortality.

The A Sydney virus strain has been circulating for three years. Is there anything
unusual about this year's strain that is making it more virulent?
So far, no laboratory data have shown the Sydney viruses circulating this season to be
substantially different from those that circulated in the previous 2 seasons. Nonetheless,
laboratories at CDC work year-round to characterize circulating influenza viruses and this
will continue.

Are people at high risk for complications from flu not getting vaccinated and might
that explain the higher mortality rates?
Every year, large numbers of people who are at risk for serious complications from influenza
-- either on the basis of age or because of underlying chronic medical conditions -- go
unvaccinated. We do not know, whether larger numbers of high-risk people than usual went
unvaccinated this year.

Is this year's vaccine working?
The match this year is very good between the circulating influenza viruses and the viruses in
the flu vaccine. Otherwise, CDC does not have data at this time from studies that directly
assess vaccine effectiveness. Results from vaccine effectiveness studies will be available
toward the end of this summer.

If there is nothing unusual about the flu strain, could something else be causing the
large number of deaths?
First, for this flu season, a somewhat broader reporting case definition was implemented in
the 122 cities mortality system. This is an important change in the methods that could be
inflating this season's P&I mortality estimates. Because the impact of the change in the
case definition is uncertain, CDC has cautioned since the early part of the season that the
P&I figures must be interpreted with caution. In addition, high levels of other
respiratory pathogens, such as respiratory syncytial virus (RSV), could also contribute to
higher levels of P&I mortality. RSV has been widespread this winter.

Should people still get a flu shot?
Any individual who is at high-risk for serious complications from influenza and who has not
been vaccinated for the 1999-2000 season should still receive flu vaccine if vaccine is
available locally. Because vaccine supply usually is limited and variable from area to area at
this time of year, individuals desiring to be vaccinated should make inquiries to their health
care providers about whether vaccine is available.

If influenza surveillance information lags by a few weeks, then how can people act on
this information?
By far, the single most important step that an individual can take to prevent flu and its
complications is to get vaccinated in the fall. Vaccination is particularly important for
people who are at high risk for complications of flu and for those who can transmit influenza
to such persons. A person should not use surveillance information to get vaccinated.

Although influenza surveillance information is useful for alerting physicians and helping
to guide treatment decisions, the primary purposes of are influenza surveillance are to
monitor changes in circulating influenza viruses, to monitor influenza activity levels and
impact, and to provide warning about pandemic viruses.

Why do some people who get the flu vaccine still get the flu?
Every year, some people who get the flu vaccine will develop influenza-like illnesses. This
can be due to various factors. First, even in the best situation, the flu vaccine is not 100%
effective in preventing illness due to influenza (see, "Is this year's vaccine
working?"). However, even when it does not prevent illness, the flu vaccine can
still prevent severe illness and death due to influenza. Second, symptoms similar to symptoms
of influenza can be caused by other viruses or bacteria. The flu vaccine will not protect a
person from illness caused by agents other than influenza viruses. The only way to be sure
that an illness is flu is to test for the influenza virus.

Does CDC recommend a second or booster vaccination for influenza?
A second vaccination (at least 1 month later) is recommended only for previously unvaccinated
children <9 years of age. Otherwise a booster vaccination is not recommended.

How many people have died, so far, during this flu season?
The average number of deaths each year from influenza in the U.S. is approximately 20,000. The
final estimated numbers of P&I deaths are based upon complete national death data. The
data for this year will not be available for about 2-3 years.